1. Field of the Invention
This invention relates in general to the measurement of pressure in a downhole submersible pump, and in particular to circuitry for compensating for DC currents in the ground.
2. Description of the Prior Art
In a submersible pump assembly, a large AC motor is located downhole for driving a centrifugal pump. AC power is supplied over three conductors extending from the surface.
Some systems employ means for detecting the pressure in the vicinity of the motor. Normally, this includes a pressure sensor located in the motor lubricant, the lubricant being at the ambient pressure in the well. The sensor is connected to a potentiometer. DC voltage is applied from the surface and superimposed on the AC lines. Filters at the surface and downhole filter out the AC. The DC voltage flows through the potentiometer which has a resistance that varies correspondingly to the pressure change.
DC currents in the ground return path can cause erroneous readings. Cathodic protection used in some wells can create the extraneous DC currents in the ground return path which effect the pressure measurement.